Add constants for the button styles.

Review Request #9439 — Created Dec. 14, 2017 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x
3654b64...

Reviewers

We had a lot of hard-coded values for button styles, which meant that
any other widgets or styles that wanted to resemble or reset these
button styles would have to duplicate those values.

This change adds constants for all these values, allowing reuse in other
stylesheets.

Tested that pages loaded and the button styles looked correct.